U.S. patent application number 10/434374 was filed with the patent office on 2004-08-26 for high performance catadioptric imaging system.
Invention is credited to Armstrong, J. Joseph, Chuang, Yung-Ho, Shafer, David R..
Application Number | 20040165257 10/434374 |
Document ID | / |
Family ID | 32871823 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040165257 |
Kind Code |
A1 |
Shafer, David R. ; et
al. |
August 26, 2004 |
High performance catadioptric imaging system
Abstract
A reduced size catadioptric objective and system is disclosed.
The objective may be employed with light energy having a wavelength
in the range of approximately 190 nanometers through the infrared
light range. Elements are less than 100 mm in diameter. The
objective comprises a focusing lens group configured to receive the
light energy and comprising at least one focusing lens. The
objective further comprises at least one field lens oriented to
receive focused light energy from the focusing lens group and
provide intermediate light energy. The objective also includes a
Mangin mirror arrangement positioned to receive the intermediate
light energy from the field lens and form controlled light energy
for transmission to a specimen. The Mangin mirror arrangement
imparts controlled light energy with a numerical aperture in excess
of 0.65 and up to approximately 0.90, and the design may be
employed in various environments.
Inventors: |
Shafer, David R.;
(Fairfield, CT) ; Chuang, Yung-Ho; (Cupertino,
CA) ; Armstrong, J. Joseph; (Milpitas, CA) |
Correspondence
Address: |
SMYRSKI & LIVESAY, LLP
3310 AIRPORT AVENUE, SW
SANTA MONICA
CA
90405
US
|
Family ID: |
32871823 |
Appl. No.: |
10/434374 |
Filed: |
May 7, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60449326 |
Feb 21, 2003 |
|
|
|
Current U.S.
Class: |
359/366 |
Current CPC
Class: |
G02B 17/0856 20130101;
G02B 17/0812 20130101; G03F 7/70341 20130101; G03F 7/70225
20130101; G02B 17/0892 20130101; G02B 21/04 20130101 |
Class at
Publication: |
359/366 |
International
Class: |
G02B 017/00; G02B
021/00; G02B 023/00 |
Claims
What is claimed is:
1. An objective employed for use with light energy having a
wavelength in the range of approximately 285 to 320 nanometers,
comprising: a focusing lens group comprising at least one focusing
lens configured to receive said light energy; a field lens oriented
to receive focused light energy from said focusing lens group and
provide intermediate light energy; and a Mangin mirror arrangement
positioned to receive the intermediate light energy from the field
lens and form controlled light energy; wherein a ratio of lens
diameter for a largest element of all focusing lenses, the field
lens, and the Mangin mirror arrangement to field size is less than
100 to 1.
2. The objective of claim 1, said Mangin mirror arrangement
comprising: a concave lens/mirror element having substantially
curved concave surfaces and second surface reflectivity; and a
relatively flat lens/mirror element having minimally curved
surfaces and second surface reflectivity.
3. The objective of claim 1, configured to have a numerical
aperture of approximately 0.9.
4. The objective of claim 1, wherein each lens has a diameter of
less than approximately 25 millimeters.
5. The objective of claim 1, wherein said objective has a field
size of approximately 0.28 millimeters.
6. The objective of claim 1, wherein said objective has a field
size of approximately 0.40 millimeters.
7. The objective of claim 1, wherein all lenses are constructed of
a single glass material.
8. The objective of claim 1, said objective used with a microscope
having a flange, wherein the flange may be located approximately 45
millimeters from the specimen, thereby providing reasonable optical
resolution.
9. The objective of claim 1, said objective used with a microscope
having a flange, wherein the flange may be located approximately
100 millimeters from the specimen, thereby providing reasonable
optical resolution.
10. The objective of claim 1, wherein said focusing lens group
comprising six focusing lenses.
11. The objective of claim 1, wherein said field lens forms an
intermediate image between said field lens and said Mangin mirror
arrangement.
12. The objective of claim 1, wherein said objective receives light
energy reflected back from said specimen through said Mangin mirror
arrangement, said field lens, and said focusing lens group.
13. The objective of claim 1, wherein ratio of lens diameter for
the largest element of all focusing lenses, the field lens, and the
Mangin mirror arrangement to field size is less than 75 to 1.
14. The objective of claim 13, wherein ratio of lens diameter for
the largest element of all focusing lenses, the field lens, and the
Mangin mirror arrangement to field size is less than 60 to 1.
15. An objective employed for use with light energy having a
wavelength in the range of approximately 157 nanometers through the
infrared light range, comprising: a focusing lens group configured
to receive said light energy and comprising at least one focusing
lens; at least one field lens oriented to receive focused light
energy from said focusing lens group and provide intermediate light
energy; and a Mangin mirror arrangement positioned to receive the
intermediate light energy from the field lens and form controlled
light energy, said Mangin mirror arrangement imparting the
controlled light energy to a specimen with a numerical aperture in
excess of 0.65, wherein each lens employed in the objective and
each element in the Mangin mirror arrangement has diameter less
than 100 millimeters.
16. The objective of claim 15, said Mangin mirror arrangement
comprising: a concave lens/mirror element having substantially
curved concave surfaces and second surface reflectivity; and a
relatively flat lens/mirror element having minimally curved
surfaces and second surface reflectivity.
17. The objective of claim 15, said objective having a field size,
wherein a of ratio lens diameter for a largest element of all
focusing lenses, each field lens, and the Mangin mirror arrangement
to field size is less than 100 to 1.
18. The objective of claim 17, wherein the of ratio lens diameter
for a largest element of all focusing lenses, each field lens, and
the Mangin mirror arrangement to field size is less than 75 to
1.
19. The objective of claim 18, wherein the of ratio lens diameter
for a largest element of all focusing lenses, each field lens, and
the Mangin mirror arrangement to field size is less than 60 to
1.
20. The objective of claim 15, wherein each lens has a diameter of
less than approximately 25 millimeters.
21. The objective of claim 15, wherein said objective has a field
size of approximately 0.28 millimeters.
22. The objective of claim 15, wherein said objective has a field
size of approximately 0.40 millimeters.
23. The objective of claim 15, wherein all lenses are constructed
of a single glass material.
24. The objective of claim 15, said objective used with a
microscope having a flange, wherein the flange may be located
proximate the specimen, thereby providing reasonable optical
resolution.
25. The objective of claim 15, said objective used with a
microscope having a flange, wherein the flange may be located
between approximately 45 and approximately 100 millimeters from the
specimen, thereby providing reasonable optical resolution.
26. The objective of claim 15, configured to have a numerical
aperture of approximately 0.9.
27. An objective constructed of a single glass material for use
with light energy having a wavelength in the range of approximately
157 nanometers through the infrared light range, comprising: at
least one focusing lens having diameter less than approximately 100
millimeters receiving said light energy and transmitting focused
light energy; at least one field lens having diameter less than
approximately 100 millimeters, receiving said focused light energy
and transmitting intermediate light energy; and at least one Mangin
mirror element having diameter less than 100 millimeters receiving
said intermediate light energy and providing controlled light
energy to a specimen.
28. The objective of claim 27, said at least one Mangin mirror
element comprising: a concave lens/mirror element having
substantially curved concave surfaces and second surface
reflectivity; and a relatively flat lens/mirror element having
minimally curved surfaces and second surface reflectivity.
29. The objective of claim 27, said objective having a field size,
wherein a of ratio lens diameter for a largest element of all
focusing lenses, each field lens, and the Mangin mirror element to
field size is less than 100 to 1.
30. The objective of claim 29, wherein the of ratio lens diameter
for a largest element of all focusing lenses, each field lens, and
the Mangin mirror element to field size is less than 75 to 1.
31. The objective of claim 30, wherein the of ratio lens diameter
for a largest element of all focusing lenses, each field lens, and
the Mangin mirror element to field size is less than 60 to 1.
32. The objective of claim 27, configured to have a numerical
aperture of approximately 0.9.
33. The objective of claim 27, wherein each lens has a diameter of
less than approximately 25 millimeters.
34. The objective of claim 27, wherein said objective has a field
size of approximately 0.28 millimeters.
35. The objective of claim 27, wherein said objective has a field
size of approximately 0.40 millimeters.
36. The objective of claim 27, said objective used with a
microscope having a flange, wherein the flange may be located
proximate the specimen and use of the objective in the flange
provides reasonable optical resolution.
37. The objective of claim 27, said objective used with a
microscope having a flange, wherein the flange may be located in a
range between approximately 45 and approximately 100 millimeters
from the specimen, thereby providing reasonable optical
resolution.
38. A system for imaging a specimen using light energy in the range
of 157 nanometers through the infrared light range, comprising: a
plurality of lenses having diameter of less than approximately 25
millimeters receiving the light energy and providing intermediate
light energy; and a Mangin mirror arrangement receiving the
intermediate light energy and providing controlled light energy to
the specimen.
39. The system of claim 38, wherein said plurality of lenses
comprise: a focusing lens group comprising a plurality of focusing
lenses, said focusing lens group receiving said light energy and
providing focused light energy; and a field lens group comprising
at least one field lens, said field lens group receiving the
focused light energy and providing the intermediate light
energy.
40. The system of claim 38, said Mangin mirror arrangement
comprising: a concave lens/mirror element having substantially
curved concave surfaces and second surface reflectivity; and a
relatively flat lens/mirror element having minimally curved
surfaces and second surface reflectivity.
41. The system of claim 39, wherein said system has a field size,
wherein a of ratio lens diameter for a largest element of all
focusing lenses, each field lens, and the Mangin mirror arrangement
to field size is less than 100 to 1.
42. The system of claim 41, wherein the of ratio lens diameter for
a largest element of all focusing lenses, each field lens, and the
Mangin mirror arrangement to field size is less than 75 to 1.
43. The system of claim 41, wherein the of ratio lens diameter for
a largest element of all focusing lenses, each field lens, and the
Mangin mirror arrangement to field size is less than 60 to 1.
44. The system of claim 38, configured to have a numerical aperture
of approximately 0.9.
45. The system of claim 38, wherein said system has a field size of
approximately 0.28 millimeters.
46. The system of claim 38, wherein said system has a field size of
approximately 0.40 millimeters.
47. The system of claim 38, said system used with a microscope
having a flange, wherein the flange may be located proximate the
specimen, thereby providing reasonable optical resolution.
48. The system of claim 38, said system used with a microscope
having a flange, wherein the flange may be located in a range
between approximately 45 and approximately 100 millimeters from the
specimen, thereby providing reasonable optical resolution.
49. A catadioptric objective comprising: a catadioptric group
comprising at least one element configured to receive light energy
from a specimen and providing reflected light energy forming
reflected light energy; a field lens group comprising at least one
field lens receiving the reflected light energy and transmitting
resultant light energy; and a focusing lens group comprising at
least one focusing lens receiving resultant light energy and
transmitting focused resultant light energy, wherein an imaging
numerical aperture for the objective is at least 0.65, the
objective having a maximum lens diameter for all lenses employed
and a field size, and wherein the ratio of maximum lens diameter to
field size is less than 100 to 1.
50. The objective of claim 49, wherein each lens is formed of fused
silica.
51. The objective of claim 49, wherein at least one lens is formed
of calcium fluoride.
52. The objective of claim 49, wherein lenses are formed from one
from a group comprising fused silica and calcium fluoride.
53. The objective of claim 49, where the catadioptric group has
less than 20 waves of on axis spherical aberration.
54. The objective of claim 53, wherein a decenter of any element by
0.01 mm reduces an associated wavefront by no more then 0.35 rms
waves at a worst field point.
55. The objective of claim 49, wherein a corrected bandwidth of the
objective is at least 10 nm.
56. The objective of claim 49, wherein a corrected bandwidth of the
objective is at least 50 nm.
57. The objective of claim 49, wherein a corrected bandwidth of the
objective includes 313 nm.
58. The objective of claim 49, wherein a corrected bandwidth of the
objective includes 266 nm.
59. The objective of claim 49, the objective having a field,
wherein a wavefront error is less than 0.1 waves RMS over the
field.
60. The objective of claim 49, the objective having a field,
wherein the field is approximately 0.28 mm in diameter.
61. The objective of claim 49, the objective having a field,
wherein the field is approximately 0.4 mm in diameter.
62. A method of imaging a specimen, comprising: focusing received
light energy using a focusing lens group; receiving focused light
energy and providing intermediate light energy using a field lens
group; and receiving intermediate light energy and forming
controlled light energy using a Mangin mirror arrangement; wherein
a field size is formed using the focusing lens group, the field
lens group, and the Mangin mirror arrangement, and wherein a ratio
of a largest element in the focusing lens group, field lens group,
and Mangin mirror arrangement to field size is less than 100 to
1.
63. The method of claim 62, said method being employed in the field
of microscopy.
64. The method of claim 62, said method being employed in the field
of semiconductor wafer inspection.
65. The method of claim 62, said method being employed in the field
of lithography.
66. The method of claim 62, said method being employed in the field
of biological inspection.
67. The method of claim 62, said method being employed in the field
of medical research.
68. The method of claim 62, wherein the ratio of the largest
element in the focusing lens group, field lens group, and Mangin
mirror arrangement to field size is less than 75 to 1.
69. The method of claim 62, wherein the ratio of the largest
element in the focusing lens group, field lens group, and Mangin
mirror arrangement to field size is less than 60 to 1.
70. The method of claim 62, wherein a largest element in the
focusing lens group, field lens group, and Mangin mirror
arrangement has diameter of less than 100 mm.
71. An objective, comprising: means for focusing received light
energy using a focusing lens group; means for receiving focused
light energy and providing intermediate light energy using a field
lens group; and means for receiving intermediate light energy and
forming controlled light energy using a Mangin mirror arrangement;
wherein a field size is formed using the focusing lens group, the
field lens group, and the Mangin mirror arrangement, and wherein a
ratio of a largest element in the focusing lens group, field lens
group, and Mangin mirror arrangement to field size is less than 100
to 1.
72. The objective of claim 71, said objective being employed in the
field of microscopy.
73. The objective of claim 71, said objective being employed in the
field of semiconductor wafer inspection.
74. The objective of claim 71, wherein the ratio of the largest
element in the focusing lens group, field lens group, and Mangin
mirror arrangement to field size is less than 75 to 1.
75. The objective of claim 71, wherein the ratio of the largest
element in the focusing lens group, field lens group, and Mangin
mirror arrangement to field size is less than 60 to 1.
76. The objective of claim 71, wherein a largest element in the
focusing lens group, field lens group, and Mangin mirror
arrangement has diameter of less than 100 mm.
77. A tube lens arrangement for use in correcting residual
aberrations in an objective, comprising: at least one focusing lens
receiving light energy and focusing preliminary light energy to an
internal field; at least one field lens in proximity to the
internal field; and at least one conversion lens receiving the
light energy from the internal field and transmitting light to a
pupil.
78. The objective of claim 1, wherein said objective has a field
size of approximately 1 millimeters.
79. The objective of claim 15, wherein said objective has a field
size of approximately 1 millimeters.
80. The objective of claim 27, wherein said objective has a field
size of approximately 1 millimeters.
81. The system of claim 38, wherein said objective has a field size
of approximately 1 millimeters.
82. The objective of claim 49, wherein said objective has a field
size of approximately 1 millimeters.
83. The objective of claim 7 where the single glass material is
fused silica.
84. The objective of claim 23 where the single glass material is
fused silica.
85. The objective of claim 23 where the single glass material is
calcium fluoride.
86. The objective of claim 1 where corrected bandwidth for the
objective is less than approximately 0.15.
87. The objective of claim 15 where corrected bandwidth for the
objective is less than approximately 0.15.
88. The objective of claim 27 where corrected bandwidth for the
objective is less than approximately 0.15.
89. The system of claim 38 where corrected bandwidth for the
objective is less than approximately 0.15.
90. The objective of claim 39 where corrected bandwidth for the
objective is less than approximately 0.15.
91. The objective of claim 77 where two glass materials are
used.
92. The objective of claim 91 where the two glass materials are
fused silica and calcium fluoride.
93. The objective of claim 77 where the tube lens is optimized to
correct the residual aberrations in a self corrected objective.
94. The objective of claim 93 where the tube lens is optimized to
increase field size.
95. The objective of claim 93 where the tube lens is optimized to
increase bandwidth.
96. The objective of claim 94 where the field size is increased
25%.
97. The objective of claim 93 where relative bandwidth is increased
to approximately 0.25.
98. The objective of claim 93 where relative bandwidth is increased
to approximately 0.35.
99. The objective of claim 77 where the objective and tube lens are
optimized together to correct for residual aberrations.
100. The objective of claim 99 where relative bandwidth is
increased to approximately 0.5.
101. The objective of claim 1, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for increased field size.
102. The objective of claim 1, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for increased relative bandwidth.
103. The objective of claim 15, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for increased field size.
104. The objective of claim 15, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for increased relative bandwidth.
105. The objective of claim 27, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for an increase in field size.
106. The objective of claim 27, the objective having a numerical
aperture associated therewith, wherein the numerical aperture is
reduced to allow for increased relative bandwidth.
107. The objective of claim 1, wherein maximum optical path error
from a 10 micron decenter of any lens is less than approximately
0.12 microns.
108. The objective of claim 1, wherein maximum optical path error
from a 10 micron decenter of each lens is employed to form an
average, and the average is less than approximately 0.08
microns.
109. The objective of claim 13 wherein maximum optical path error
from a 10 micron decenter of any lens is less than approximately
0.12 microns.
110. The objective of claim 13 wherein maximum optical path error
from a 10 micron decenter of each lens is employed to form an
average, and the average is less than approximately 0.08
microns.
111. The objective of claim 27 wherein maximum optical path error
from a 10 micron decenter of any lens is less than approximately
0.12 microns.
112. The objective of claim 27 wherein maximum optical path error
from a 10 micron decenter of each lens is employed to form an
average and the average is less than approximately 0.08
microns.
113. The system of claim 38 wherein maximum optical path error from
a 10 micron decenter of any lens is less than approximately 0.12
microns.
114. The system of claim 38 wherein maximum optical path error from
a 10 micron decenter of each lens is employed to form an average
and the average is less than approximately 0.08 microns.
115. The objective of claim 49 wherein maximum optical path error
from a 10 micron decenter of any lens is less than approximately
0.12 microns.
116. The objective of claim 49 wherein maximum optical path error
from a 10 micron decenter of each lens is employed to form an
average and the average is less than approximately 0.08
microns.
117. An objective employed for use with light energy having a
wavelength in the range of approximately 157 nanometers through the
infrared light range, comprising: focusing means for receiving said
light energy and providing focused light energy; field lensing
means for receiving focused light energy from said focusing means
and providing intermediate light energy; and mirror means for
receiving the intermediate light energy from the field lensing
means and forming controlled light energy, said mirror means
imparting the controlled light energy to a specimen with a
numerical aperture in excess of 0.65, wherein each lens employed in
the objective and each element in the mirror means has diameter
less than 100 millimeters.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application Serial No. 60/449,326, entitled "High
Performance, Low Cost Catadioptric Imaging System," filed Feb. 21,
2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to the field of
optical imaging and more particularly to catadioptric optical
systems used for microscopic imaging, inspection, and lithography
applications.
[0004] 2. Description of the Related Art
[0005] Many optical and electronic systems exist to inspect surface
features for defects such as those on a partially fabricated
integrated circuit or a photomask. Defects may take the form of
particles randomly localized on the surface, scratches, process
variations, and so forth. Such techniques and devices are well
known in the art and are embodied in various commercial products
such as those available from KLA-Tencor Corporation of San Jose,
Calif.
[0006] Specialized optical systems are required in inspection
devices to enable imaging of defects found on semiconductor wafers
and photomasks. Improved performance for such systems may be
realized using specially designed components that facilitate
beneficial inspection parameters, such as high numerical apertures.
The numerical aperture of an objective represents the objective's
ability to collect light and resolve fine specimen detail at a
fixed object distance. Numerical aperture is measured as the sine
of the vertex angle of the largest cone of meridional rays that can
enter or leave the optical system or element, multiplied by the
refractive index of the medium in which the vertex of the cone is
located. A large numerical aperture provides distinct advantages
during inspection, not the least of which is an ability to resolve
smaller features of the specimen. Also, high NAs collect a larger
scattering angle, thereby tending to improve performance in
darkfield environments over systems having relatively low NAs. Two
patents that disclose high numerical aperture (NA) catadioptric
systems are U.S. Pat. No. 5,717,518 to Shafer et al. and U.S. Pat.
No. 6,483,638 to Shafer et al. A representative illustration of a
catadioptric design 100 in accordance with the teachings of the
'518 patent is presented in FIG. 1, which is similar to FIG. 1 of
the '518 patent. A representative illustration of a catadioptric
design 200 in accordance with the teachings of the '638 patent is
presented in FIG. 2, which has similarities to FIG. 4 of the '638
patent.
[0007] U.S. Pat. No. 5,717,518 to Shafer et al. discloses an
imaging design capable of high NA, ultra broadband UV imaging. The
high NA (up to approximately 0.9) system can be used for broadband
bright field and multiple wavelength dark-field imaging. Certain
issues exist with designs similar to that presented in FIG. 1.
First, the field lens group may need to be physically located
within a central hole in the large curved catadioptric element,
which can make manufacturing very difficult and extremely
expensive. Second, the field lens elements in such a design may
require at least one glued interface. In the presence of
wavelengths less then 365 nm, reliable glues that can withstand
light intensity levels at an internal focus are generally
unavailable. Third, the lens elements in such a design may be
located very close to a field plane, thereby requiring a high
degree of, or nearly perfect, surface quality and bulk material
quality to prevent image degradation. Fourth, element diameters are
typically larger than a standard microscope objective, especially
for the catadioptric group. Large diameter elements frequently make
integration into an inspection system difficult and can increase
manufacturing costs.
[0008] The design of FIG. 2 is generally capable of high NA, ultra
broadband UV imaging. The design is a high NA (up to approximately
0.9) imaging system that can be used for broadband bright field and
multiple wavelength dark-field imaging and can use a varifocal tube
lens to provide a large range of magnifications. The FIG. 2 design
introduces very tight tolerances in the field lens group, due in
part to increased on-axis spherical aberration produced by the
catadioptric group. This on-axis spherical aberration must be
corrected by the following refractive lens elements. The design of
FIG. 2 is relatively large, thereby generally requiring complicated
optomechanical mounting of elements, especially in the catadioptric
group.
[0009] Other optical arrangements have been developed to perform
specimen inspection, but each arrangement tends to have certain
specific drawbacks and limitations. Generally, in a high precision
inspection environment, an objective with a short central
wavelength provides advantages over those with long central
wavelengths. Shorter wavelengths can enable higher optical
resolution and improved defect detection, and can facilitate
improved defect isolation on upper layers of multi-layer specimens,
such as semiconductor wafers. Shorter wavelengths can provide
improved defect characterization. An objective that can cover as
large a wavelength range as possible may also be desirable,
particularly when using an arc lamp as an illumination source. An
all refractive objective design is difficult in this wavelength
range because few glass materials having high transmission are
effective for chromatic correction. A small bandwidth may not be
desirable for inspection applications due to limitation of
available light power and increased interference from thin films on
the surface being inspected.
[0010] A large field size can provide distinct advantages during
inspection. One advantage is an ability to scan a larger area of
the specimen in a given period of time, thereby increasing
throughput, measured as the ability to scan a large area over a
small period of time. A relatively large field size in a typical
design in this type of environment can be approximately or greater
than 0.2 mm using an imaging magnification of 200.times. to support
a sensor with an 40 mm diagonal. Small objectives are also
desirable, as small objectives can be used in combination with
standard microscope objectives and fit in standard microscope
turrets. The standard objective flange to object length is 45 mm,
while certain objectives employ lens diameters greater than 100 mm
having length of over 100 mm. Other smaller catadioptric objectives
have been produced, but still typically have lens diameters in
excess of 60 mm and length over 60 mm. Certain of these smaller
objectives have NAs limited to 0.75 and field sizes limited to 0.12
mm with a bandwidth less than 10 nm. Such designs typically use a
Schwartzchild approach with lenses added within the catadioptric
group in an effort to improve performance. Working distances are
typically greater than 8 mm. This design approach can somewhat
reduce the objective diameter, at the cost of increasing central
obscuration, significantly degrading objective performance.
[0011] An objective having low intrinsic aberrations is also
desirable, as is an objective that is largely self-corrected for
both monochromatic and chromatic aberrations. A self corrected
objective will have looser alignment tolerances with other self
corrected imaging optics. An objective with loose manufacturing
tolerances, such as lens centering tolerances, may be particularly
beneficial. Further, reducing incidence angles on lens surfaces can
have a large effect on optical coating performance and
manufacturing. In general, lower angles of incidence on lens
surfaces also produce looser manufacturing tolerances.
[0012] It would be beneficial to provide a system overcoming these
drawbacks present in previously known systems and provide an
optical inspection system design having improved functionality over
devices exhibiting those negative aspects described herein.
SUMMARY OF THE INVENTION
[0013] According to a first aspect of the present design, there is
provided an objective employed for use with light energy having a
wavelength in the range of approximately 285 to 320 nanometers. The
objective comprises a focusing lens group comprising at least one
focusing lens configured to receive the light energy, a field lens
oriented to receive focused light energy from the focusing lens
group and provide intermediate light energy, and a Mangin mirror
arrangement positioned to receive the intermediate light energy
from the field lens and form controlled light energy. A ratio of
lens diameter for a largest element of all focusing lenses, the
field lens, and the Mangin mirror arrangement to field size is less
than 100 to 1.
[0014] According to a second aspect of the present design, there is
provided an objective employed for use with light energy having a
wavelength in the range of approximately 157 nanometers through the
infrared light range. The objective comprises a focusing lens group
configured to receive the light energy and comprising at least one
focusing lens, at least one field lens oriented to receive focused
light energy from the focusing lens group and provide intermediate
light energy, and a Mangin mirror arrangement positioned to receive
the intermediate light energy from the field lens and form
controlled light energy. The Mangin mirror arrangement imparts
controlled light energy to a specimen with a numerical aperture in
excess of 0.65, wherein each lens employed in the objective and
each element in the Mangin mirror arrangement has diameter less
than 100 millimeters.
[0015] According to a third aspect of the present design, there is
provided an objective constructed of a single glass material for
use with light energy having a wavelength in the range of
approximately 157 nanometers through the infrared light range. The
objective comprises at least one focusing lens having diameter less
than approximately 100 millimeters receiving the light energy and
transmitting focused light energy, at least one field lens having
diameter less than approximately 100 millimeters, receiving the
focused light energy and transmitting intermediate light energy,
and at least one Mangin mirror element having diameter less than
100 millimeters receiving the intermediate light energy and
providing controlled light energy to a specimen.
[0016] According to a fourth aspect of the present design, there is
provided a system for imaging a specimen using light energy in the
range of 157 nanometers through the infrared light range. The
system comprises a plurality of lenses having diameter of less than
approximately 25 millimeters receiving the light energy and
providing intermediate light energy, and a Mangin mirror
arrangement receiving the intermediate light energy and providing
controlled light energy to the specimen.
[0017] According to a fifth aspect of the present design, there is
provided a catadioptric objective comprising a catadioptric group
comprising at least one element configured to receive light energy
from a specimen and providing reflected light energy forming
reflected light energy, a field lens group comprising at least one
field lens receiving the reflected light energy and transmitting
resultant light energy, and a focusing lens group comprising at
least one focusing lens receiving resultant light energy and
transmitting focused resultant light energy, wherein an imaging
numerical aperture for the objective is at least 0.65, the
objective having a maximum lens diameter for all lenses employed
and a field size, and wherein the ratio of maximum lens diameter to
field size is less than 100 to 1.
[0018] According to a sixth aspect of the present design, there is
provided a method of imaging a specimen. The method comprises
focusing received light energy using a focusing lens group,
receiving focused light energy and providing intermediate light
energy using a field lens group, and receiving intermediate light
energy and forming controlled light energy using a Mangin mirror
arrangement. A field size is formed using the focusing lens group,
the field lens group, and the Mangin mirror arrangement, and a
ratio of a largest element in the focusing lens group, field lens
group, and Mangin mirror arrangement to field size is less than 100
to 1.
[0019] According to a seventh aspect of the present design, there
is provided an objective comprising means for focusing received
light energy using a focusing lens group, means for receiving
focused light energy and providing intermediate light energy using
a field lens group, and means for receiving intermediate light
energy and forming controlled light energy using a Mangin mirror
arrangement. A field size is formed using the focusing lens group,
the field lens group, and the Mangin mirror arrangement, and a
ratio of a largest element in the focusing lens group, field lens
group, and Mangin mirror arrangement to field size is less than 100
to 1.
[0020] According to an eighth aspect of the present design, there
is provided an objective employed for use with light energy having
a wavelength in the range of approximately 157 nanometers through
the infrared light range. The objective comprises focusing means
for receiving the light energy and providing focused light energy,
field lensing means for receiving focused light energy from the
focusing means and providing intermediate light energy, and mirror
means for receiving the intermediate light energy from the field
lensing means and forming controlled light energy, the mirror means
imparting the controlled light energy to a specimen with a
numerical aperture in excess of 0.65, wherein each lens employed in
the objective and each element in the mirror means has diameter
less than 100 millimeters.
[0021] These and other objects and advantages of the present
invention will become apparent to those skilled in the art from the
following detailed description of the invention and the
accompanying drawings.
DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 illustrates an aspect of the catadioptric objective
design similar to that presented in FIG. 1 of U.S. Pat. No.
5,717,518;
[0023] FIG. 2 is an aspect of the catadioptric objective design
similar to that presented in FIG. 4 of U.S. Pat. No. 6,483,638;
[0024] FIG. 3 presents a reduced size catadioptric objective with a
high NA in accordance with the present invention;
[0025] FIG. 4 is a size comparison between the objective of the
design presented in FIG. 2, the reduced size catadioptric objective
of FIG. 3, and a standard microscope objective;
[0026] FIG. 5A is a graph of decenter sensitivity for the objective
design of FIG. 3;
[0027] FIG. 5B presents a sensitivity comparison between the design
of FIG. 3, a UV refractive objective corrected from 365-436 nm, and
a catadioptric objective based on the design of FIG. 2;
[0028] FIG. 6 is an alternate aspect of the reduced size
catadioptric objective in accordance with the present
invention;
[0029] FIG. 7A illustrates a tube lens arrangement in accordance
with the present invention;
[0030] FIG. 7B illustrates an objective and tube lens corrected
from 266-405 nm with a 0.4 mm diameter field;
[0031] FIG. 7C illustrates an objective design with a 1 mm field
size and diameter of approximately 58 mm;
[0032] FIG. 8 is a design able to perform in the presence of
wavelengths from approximately 311-315 nm, having approximately 26
mm diameter, a field size of approximately 0.28 mm, and NA of
approximately 0.90;
[0033] FIG. 9 is an approximately 0.28 mm field design having
approximately 26 mm diameter, a wavelength of between approximately
297 and 313 nm, and NA of approximately 0.90;
[0034] FIG. 10 is an approximately 0.4 mm field design having
approximately 26 mm diameter, a wavelength of between approximately
297 and 313 nm, and NA of approximately 0.90;
[0035] FIG. 11 illustrates a broad band design having approximately
26 mm diameter, a wavelength of between approximately 266 and 313
nm, field size of approximately 0.28 mm, and NA of approximately
0.90; and
[0036] FIG. 12 is a graph comparing relative bandwidth versus the
maximum lens element diameter of certain designs, including the
current design; and
[0037] FIG. 13 is a graph comparing field size versus the maximum
lens element diameter of certain designs, including the present
design.
DETAILED DESCRIPTION OF THE INVENTION
[0038] According to the present invention, there is provided a
catadioptric objective corrected over a wavelength range from
285-320 nm using a single glass material, or in certain
circumstances, more than one glass material to improve performance.
One aspect of the objective design is shown in FIG. 3. The
catadioptric objective as shown in FIG. 3 is optimized for
broad-band imaging in the UV spectral region, namely approximately
0.285 to 0.320 micron wavelengths. The objective provides
relatively high numerical apertures and large object fields. The
inventive design presented uses the Schupmann principle in
combination with an Offner field lens to correct for axial color
and first order lateral color. As shown in the aspect presented in
FIG. 3, the field lens group 305 is slightly displaced from the
intermediate image 306 to obtain enhanced performance.
[0039] From FIG. 3, the catadioptric group 301 or Mangin mirror
arrangement includes a Mangin mirror element 302. Mangin mirror
element 302 is a reflectively coated lens element. The catadioptric
group 301 also includes and a concave spherical reflector 303, also
a reflectively coated lens element. Both elements in the
catadioptric group 301 have central optical apertures where
reflective material is absent. This allows light to pass from the
object or specimen 300 (not shown) through Mangin mirror element
302, reflect from the second or inner surface of concave spherical
reflector 303, onto the reflective surface 320 of Mangin mirror
element 302, and through concave spherical reflector 303 to form an
intermediate image 306 between concave spherical reflector 303 and
field lens group 304. The field lens group 304 may comprise one or
more lenses, and in the aspect shown in FIG. 3, one field lens is
employed in the field lens group 304.
[0040] The focusing lens group 307 uses multiple lens elements, in
the aspect shown six lens elements 308, 309, 310, 311, 312, and
313. All lenses in the focusing lens group 307 may be formed from a
single type of material to collect the light from the field lens
group 304 and the intermediate image 306.
[0041] The lens prescription for the aspect of the invention
illustrated in FIG. 3 is presented in Table 1.
1TABLE 1 Prescription for lenses for the design of FIG. 3 Surface
Number Radius Thickness Glass Diameter OBJ Infinity Infinity 0 1
Infinity 15.50165 9.39467 STO Infinity -15.50165 8 3 53.51878 2
Fused Silica 9.376161 4 -18.17343 0.976177 9.234857 5 10.48757
1.249953 Fused Silica 8.151456 6 5.891816 3.328088 7.199539 7
-5.254784 5.105439 Fused Silica 7.084075 8 -8.860388 0.5 9.430437 9
12.82516 2 Fused Silica 9.711337 10 61.04848 0.5 9.468601 11
8.892555 1.75 Fused Silica 9.125279 12 15.75614 2.126452 8.563035
13 7.216376 2 Fused Silica 7.4431 14 21.90145 5.382485 6.702302 15
2.321495 1.3 Fused Silica 2.530266 16 13.47255 0.669203 1.651874 17
Infinity 0.498865 0.711891 18 17.99728 3.170995 Fused Silica 25 19
13.41607 6.08537 21 20 972.9414 5.220004 Fused Silica 20.5 21 -78
-5.220004 MIRROR 20.5 22 972.9414 -6.08537 20.5 23 13.41607
-3.170995 Fused Silica 21 24 17.99728 3.170995 MIRROR 25 25
13.41607 6.08537 21 26 972.9414 5.220004 Fused Silica 20.5 27 -78
0.3 20.5 IMA Infinity 0.410191
[0042] As may be appreciated by one skilled in the art, the numbers
in the leftmost column of Table 1 represent the surface number
counting surfaces from the left of FIG. 3. For example, the left
surface of lens 308 in the orientation presented in FIG. 3 (surface
3 in Table 1) has a radius of curvature of 53.51878 mm, a thickness
of 2 mm, and the rightmost surface (surface 4) has a radius of
curvature of -18.17343 mm, and is 0.976177 mm from the next
surface. The material used is fused silica, and the diameter of the
left surface is 9.376161 mm and of the right surface is 9.234857
mm.
[0043] In the design presented in FIG. 3, the numerical aperture
may approach or even exceed approximately 0.90. The design
presented herein, including the aspect illustrated in FIG. 3,
provides a maximum numerical aperture in all cases in excess of
0.65.
[0044] From FIG. 3, the focusing lens group 307 has the ability to
receive light energy and transmit focused light energy. The field
lens group 304 has the ability to receive the focused light energy
and provide intermediate light energy, and form intermediate image
306. The catadioptric group or Mangin mirror arrangement 301
receives the intermediate energy and provides controlled light
energy to the specimen. Alternately, the reflected path originates
at the specimen, and light reflected from the specimen is received
by the catadioptric group or Mangin mirror arrangement 301 and
forms and transmits reflected light energy. The field lens group
304 receives the reflected light energy and transmitting resultant
light energy, and the focusing lens group receives resultant light
energy and transmits focused resultant light energy.
[0045] The design presented in FIG. 3 and Table 1 thus uses a
single glass material, fused silica. Other materials may be
employed, but it is noted that fused silica or any material used
within the design may require low absorption over a wide range of
wavelengths from 190 nm through the infrared wavelength. Use of
fused silica can enable the design to be re-optimized for any
center wavelength in this wavelength range. For example, the design
can be optimized for use with lasers at 193, 198.5, 213, 244, 248,
257, 266, 308, 325, 351, 355, or 364nm. The design can also be
optimally employed to cover lamp spectral bands from 192-194,
210-216, 230-254, 285-320, and 365-546 nm. In addition, if calcium
fluoride is employed as the glass or lens material, the design can
be employed with an excimer laser at 157 nm or excimer lamps at 157
or 177 nm. Re-optimization requires tuning or altering components
within the abilities of those skilled in the art. Calcium fluoride
lenses may also be employed in the field lens group to increase the
bandwidth of the objective, a modification discussed generally in
U.S. Pat. No. 5,717,518.
[0046] As noted in FIG. 3, the objective has a diameter of 26
millimeters, which is significantly smaller than objectives
previously employed in this wavelength range. The small size of
this objective is particularly beneficial in view of the
performance characteristics of the objective. The objective can be
mounted in a standard microscope turret with a 45 mm
flange-to-object separation. The objective supports a numerical
aperture of approximately 0.90, a field size of approximately 0.4
mm, has a corrected bandwidth from approximately 285-313 nm, and a
polychromatic wavefront error of less than approximately 0.038
waves. A size comparison between the objective of the design
presented in FIG. 2, the design of FIG. 3, and a standard
microscope objective is shown in FIG. 4.
[0047] As is true with any optical design, certain tradeoffs may be
made to improve performance characteristics depending on the
desired application of the objective or optical design. It is
possible, for example, to sacrifice bandwidth, field size,
numerical aperture, and/or objective size to enhance one of the
aforementioned performance characteristics, depending on the
application. For example, optimizing for lower or higher NAs is
possible. Reducing the NA can reduce the manufacturing tolerance
and the outer diameter of the objective. Lower NA designs can
provide larger field sizes and larger bandwidths. Lower NA designs
with the same performance and less optical elements are also
possible. Optimizing for higher NAs is also possible. Optimizing
the design for higher NAs would generally limit the field size or
bandwidth and may require slightly increased diameter objective
elements.
[0048] The design of FIG. 3 has a field size of 0.4 mm in diameter.
Such a relatively large field size can support a large high speed
sensor. For example, using an imaging magnification of 200.times.,
a sensor having an 80 mm diagonal can be supported. The design of
FIG. 3 can also be extended to larger field sizes by allowing
larger lens diameters and re-optimizing the elements, again a task
within the range of those skilled in the art.
[0049] The design of FIG. 3 has a relatively low intrinsic
polychromatic wavefront aberration over the design bandwidth from
approximately 285-320 nm. The low wavefront aberration provides
increased manufacturing headroom, or ease of manufacture, while
enabling relatively high performance of the manufactured objective.
The design of FIG. 3 provides good performance over narrow bands
from approximately 266 to 365 nm if the objective is refocused,
again a task that may be readily performed by one of ordinary skill
in the art. Use of the objective of FIG. 3 in this narrow band
range allows use of lasers or narrow lamp spectra, such as the 365
nm line of lasers. The design is also self corrected. Self
corrected in this context means that the objective does not require
any additional optical components to correct aberrations in order
to achieve the design specifications. The ability to self-correct
tends to simplify optical testing metrology and optical alignment
to other self corrected imaging optics. Further correction of
residual aberrations using additional imaging optics is also
possible. Further aberration correction can increase optical
specifications such as bandwidth or field size.
[0050] One advantage of the present design is relatively loose
manufacturing tolerances. Specifically, the decenter tolerances of
individual lenses are relatively loose. Having loose decenter
tolerances for individual lens elements tends to simplify the
manufacturing requirements of the system. Any lens decenters
encounterd during manufacturing may cause on-axis coma, a
phenomenon that can be difficult to compensate without introducing
other residual aberrations. Using the present design, it is
possible to reduce the decenter sensitivity of the lens and mirror
elements by carefully balancing the aberrations within the
catadioptric group 301 and focusing lens group 307. Total
aberrations of the catadioptric group may be optimized using the
design of FIG. 3 to balance the compensation required by the field
lens group 304 and focusing lens group 307. FIG. 5A shows the
decenter sensitivity for the objective. A 10 micron decenter,
without using any compensators, introduces less than approximately
0.2 waves of aberration in all elements except lens 302. A 10
micron decenter without compensators introduces approximately 0.29
waves of aberration. In the design presented in FIG. 3, average
tolerance is approximately 0.13 waves of error at approximately 313
nm. Further balancing the tolerances on the elements in the
catadioptric group 301 is also possible.
[0051] The decenter tolerances also scale with the wavelength being
used. This is because the optical path errors introduced for small
decenters are not a strong function of wavelength. For example, if
a 10 micron decenter introduces 0.2 waves of aberration at a 266 nm
wavelength, this is equivalent to a 0.0532 micron optical path
error. The system operating at 365 nm would only introduce 0.15
waves of aberration for the same decenter. This would have the same
0.0532 micron optical path error.
[0052] These tolerances tend to be looser than other catadioptric
designs in similar environments, and tend to be looser than most
standard refractive objective designs. FIG. 5B presents a
sensitivity comparison between the design of FIG. 3, a UV
refractive objective corrected from 365-436 nm, and a catadioptric
objective based on the design shown in FIG. 2. Generally, in the
plots of FIG. 5B, a value lower on the vertical scale indicates a
more desirable design. Wavefront error is plotted for a 10 micron
decenter for each element without compensation. Average sensitivity
is less than the refractive objective and much less than the
sensitivity of the catadioptric objective design similar to that
from the design of FIG. 2.
[0053] This design also has very loose tolerances on the index of
the glass material. This is largely because the design is of a
single material and does not rely on the index difference of two
different glass materials to compensate for chromatic aberrations.
This also makes the design very insensitive to temperature changes.
Standard designs use multiple glass materials with different index
profiles for color correction. The index profile for each material
changes differently with temperature. This changes the chromatic
correction for temperatures other than the design temperature and
reduces the performance.
[0054] The objective design presented herein can support various
modes of illumination and imaging. Modes supported can include
bright field and a variety of dark field illumination and imaging
modes. Other modes such as confocal, differential interference
contrast, polarization contrast may also be supported using the
present design.
[0055] Bright field mode is commonly used in microscope systems.
The advantage of bright field illumination is the clarity of the
image produced. Using bright field illumination with an objective
such as that presented herein provides a relatively accurate
representation of object feature size multiplied by the
magnification of the optical system. The objective and optical
components presented herein can be readily used with image
comparison and processing algorithms for computerized object
detection and classification. Brightfield mode typically uses a
broad band incoherent light source, but it may be possible to use
laser illumination sources using slightly modified illumination
system components employing the design presented herein.
[0056] The confocal mode has been used for optical sectioning to
resolve height differences of object features. Most imaging modes
have difficulty detecting changes in the height of features. The
confocal mode forms separate images of object features at each
height of interest. Comparison of the images then shows the
relative heights of different features. Confocal mode may be
employed using the design presented herein.
[0057] Dark field mode has been used to detect features on objects.
The advantage of the dark field mode is that flat specular areas
scatter very little light toward the detector, resulting in a dark
image. Surface features or objects protruding above the object tend
to scatter light toward the detector. Thus, in inspecting objects
like semiconductor wafers, dark field imaging produces an image of
features, particles, or other irregularities on a dark background.
The present design may be employed with dark field mode
illumination. Dark field mode provides a large resultant signal
upon striking small features that scatter light. This large
resultant signal allows larger pixels to be employed for a given
feature size, permitting faster object inspections. Fourier
filtering can also be used to minimize the repeating pattern signal
and enhance the defect signal to noise ratio during dark field
inspection.
[0058] Many different dark field modes exist, each including a
specific illumination and collection scheme. Illumination and
collection schemes can be chosen such that the scattered and
diffracted light collected from the object provides an acceptable
signal-to-noise ratio. Certain optical systems use different dark
field imaging modes including ring dark field, laser directional
dark field, double dark field, and central dark ground. Each of
these dark field imaging modes may be employed in the present
design.
[0059] An alternate aspect of the present design presents an
objective with increased bandwidth. This aspect of the design is
presented in FIG. 6. The main difference between the design of FIG.
6 and that of FIG. 3 is the tradeoff between bandwidth and field
size. The objective of the design of FIG. 6 is corrected over a
broader bandwidth from 266 to 320 nm but has a relatively smaller
field, approximately 0.28 mm, as compared with the 0.4 mm of the
design of FIG. 3. The design of FIG. 6 maintains the high
approximately 0.90 numerical aperture. The worst case polychromatic
wavefront error for the FIG. 6 design is approximately 0.036
waves.
[0060] From FIG. 6, the catadioptric group 601 includes a Mangin
mirror element 602, which is a reflectively coated lens element,
and a concave spherical reflector 603, which is also a reflectively
coated lens element. Both Mangin mirror element 602 and concave
spherical reflector 603 have central optical apertures where
reflective material is absent. The absence of reflective material,
in the center of the components shown, allows light to pass from
the object or specimen 600 (not shown) through Mangin mirror
element 602, reflect from the second surface of concave spherical
reflector 603 onto the Mangin mirror element 602, and transmit
through concave spherical reflector 603 to form an intermediate
image 620 between concave spherical reflector 603 and field lens
group 604, comprising a single field lens 615 in this aspect of the
design.
[0061] The focusing lens group 605 employs multiple lens elements,
in this aspect the six lens elements 606, 607, 608, 609, 610, and
611, which may all be formed from a single type of material. The
focusing lens group 605 collects light from the field lens group
604, including the intermediate image 620.
[0062] The design presented in FIG. 6 has the advantages and
flexibility described with respect to the design of FIG. 3. The
lens prescription for this embodiment is shown in Table 2.
2TABLE 2 Prescription for lenses for the design of FIG. 6 Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0 1 Infinity
16.20723 9.020484 STO Infinity -16.20723 8 3 64.63011 2 FUSED
SILICA 9.010584 4 -19.00905 1.675169 8.894847 5 10.3536 1.249991
FUSED SILICA 7.776084 6 5.91317 3.249904 6.942948 7 -5.240171
5.243182 FUSED SILICA 6.855225 8 -9.11876 0.5 9.288367 9 16.20784 2
FUSED SILICA 9.638653 10 Infinity 0.5 9.499901 11 8.951438 3.573584
FUSED SILICA 9.210718 12 12.83071 0.5 7.808034 13 7.107306 2 FUSED
SILICA 7.502914 14 29.37779 5.583862 6.837774 15 2.252897 1.3 FUSED
SILICA 2.391106 16 11.8636 0.668164 1.486574 17 Infinity 0.499742
0.548495 18 17.95894 3.09472 FUSED SILICA 25 19 13.41421 6.156826
21 20 1134 5.204856 FUSED SILICA 20.5 21 -78 -5.204856 MIRROR 20.5
22 1134 -6.156826 20.5 23 13.41421 -3.09472 FUSED SILICA 21 24
17.95894 3.09472 MIRROR 25 25 13.41421 6.156826 21 26 1134 5.204856
FUSED SILICA 20.5 27 -78 0.3 20.5 IMA Infinity 0.289101
[0063] A further aspect of the present design uses a tube lens to
correct for residual aberrations in the objective. Residual
aberrations are primarily the chromatic variation of distortion and
higher order lateral color. These residual aberrations are related
to use of the Offner field lens in the objective. One method to
correct these residual aberrations is to employ a second glass
material in the Offner field lens. Use of a second material can
lead to an optical design with large elements and relatively tight
tolerances. The alternative approach presented in this design is to
use additional imaging optics to correct for residual aberrations.
Such a design can produce a system having high NA, large field
size, small lens diameter, as well as relatively loose
tolerances.
[0064] Correcting these residual aberrations can further increase
the field size or increase the bandwidth while maintaining the
field size. The design of FIG. 7A maintains the same approximately
0.4 mm field size as in the design of FIG. 3 and extends the
bandwidth to cover 266 to 365 nm without need for refocusing. The
worst case polychromatic wavefront error for the design of FIG. 7A
is approximately 0.036 waves.
[0065] The design includes two air spaced doublets 701, 702, 703,
and 704, the doublets 701-704 fashioned from fused silica and
calcium fluoride. The doublets 701-704 focus light through three
fused silica lens elements, namely lens elements 705, 706, and 707.
these lens elements 705-707 are in proximity to an internal field.
Light is then collimated by an air spaced triplet 708, 709, and
710. Light then forms an external pupil at 711. The external pupil
711 can be used for placing dark field apertures, Fourier filters,
and beamsplitters.
[0066] The lens prescription for the aspect of the invention
illustrated in FIG. 7A is shown in Table 3.
3TABLE 3 Prescription for lenses for the design of FIG. 7A Surf
Radius Thickness Glass Diameter OBJ Infinity 0.3 0.4 1 78 5.155765
FUSED SILICA 21 2 -1031.094 6.132752 21 3 -13.38766 3.334036 FUSED
SILICA 21.5 4 -18.2281 -3.334036 MIRROR 25.5 5 -13.38766 -6.132752
21.5 6 -1031.094 -5.155765 FUSED SILICA 21 7 78 5.155765 MIRROR 21
8 -1031.094 6.132752 21 9 -13.38766 3.334036 FUSED SILICA 21.5 10
-18.2281 0.598511 25.5 11 Infinity 0.595647 0.87265 12 -22.67364
1.496994 FUSED SILICA 1.716759 13 -2.487035 5.332021 2.721696 14
-24.12325 1.749722 FUSED SILICA 6.761726 15 -8.563906 1.647307
7.426322 16 Infinity 1.017137 8.707626 17 -23.20559 1.75 FUSED
SILICA 9.034138 18 -10.09888 0.499806 9.544791 19 459.357 2 FUSED
SILICA 10.00487 20 -12.90167 0.499731 10.16545 21 9.888518 5.284916
FUSED SILICA 9.738469 22 5.468369 3.606566 7.299015 23 -6.158311
1.499744 FUSED SILICA 7.434168 24 -10.89758 0.499623 8.474502 25
18.52911 2 FUSED SILICA 9.287792 26 -68.1321 -15.25736 9.417208 STO
Infinity 15.25736 8.09706 28 Infinity 34.89506 9.431455 29 -- 0 --
30 Infinity 6 FUSED SILICA 18.46143 31 Infinity 0 25.39024 32 -- 0
--- 33 Infinity 30 17.16851 34 -- 0 -- 35 Infinity 6 FUSED SILICA
26.81778 36 Infinity 0 20.63295 37 -- 0 -- 38 Infinity 81 15.2277
39 -159.7003 4 FUSED SILICA 22.27788 40 37.47386 0.999856 22.92295
41 33.36497 9 CAF2 23.58799 42 -80.14523 3.436442 24.07579 43
38.4464 8 CAF2 23.97432 44 -53.0633 1.5 22.95647 45 -39.45511 3
FUSED SILICA 22.35342 46 1094.058 43.27621 21.67501 47 10.8487
3.18507 FUSED SILICA 12.40192 48 8.96916 4.999989 10.71199 49
-24.58978 2.5 FUSED SILICA 10.26452 50 117.1346 47.95638 10.34545
51 175.9777 5 FUSED SILICA 16.71625 52 -37.37344 74.18151 17.10185
53 -1113.4 5 CAF2 11.5593 54 -14.94822 0.99955 11.38304 55 -13.4032
2 FUSED SILICA 10.93698 56 18.26209 0.99969 10.92178 57 17.51017 6
CAF2 11.25199 58 -33.75194 38.51994 11.218 59 -- 100 4.910667 IMA
Infinity 16.34334
[0067] The tube lens design of FIG. 7A uses only fused silica and
calcium fluoride. Both of these materials have transmissions from
approximately 190 nm through the infrared. Thus a tube lens can be
designed to operate with an objective that can be re-optimized for
different center wavelengths. Other tube lens magnifications may be
achieved using this design. The design of FIG. 7A can be
re-optimized for different afocal magnifications depending on the
desired overall magnification. Using the design presented herein, a
focusing tube lens that directly forms an image that can expose a
high speed sensor may be realized.
[0068] An additional aspect of the present design uses a tube lens
to correct for residual aberrations in the objective. Correcting
these residual aberrations can increase the field size or increase
the bandwidth while maintaining the field size. Residual
aberrations are primarily the chromatic variation of distortion and
higher order lateral color. The design of FIG. 7B maintains the
same approximately 0.4 mm field size as in the design of FIG. 3 and
extends the bandwidth to cover 266 to 405 nm without need for
refocusing. The worst case polychromatic wavefront error for the
design of FIG. 7B is approximately 0.041 waves.
[0069] The design in FIG. 7B is composed of an objective 751 that
collects the light and a tube lens 753 that corrects residual
aberations. A set of lenses 752 may be provided to enhance
performance. To achieve additional bandwidth beyond the design of
FIG. 7A, the objective and tube lens of FIG. 7B are partially
optimized together. Partial combined optimization of the objective
and tube lens allows for further correction of the limiting off
axis lateral color and chromatic variation of distortion. The tube
lens forms an external pupil 754 that can be used in the same
fashion as the design of FIG. 7A. The design presented in FIG. 7B
also shows optional beamsplitter elements 754 that can be used to
fold in illumination and autofocus light. The lens prescription for
the aspect of the invention illustrated in FIG. 7B is shown in
Table 4.
4TABLE 4 Prescription for lenses for the design of FIG. 7B Surf
Radius Thickness Glass Diameter OBJ Infinity 0.300 0.4 1 78.000
5.168 Fused silica 21 2 -850.121 6.031 21 3 -13.361 3.505 Fused
silica 21.5 4 -18.352 -3.505 MIRROR 25.5 5 -13.361 -6.031 21.5 6
-850.121 -5.168 Fused silica 21 7 78.000 5.168 MIRROR 21 8 -850.121
6.031 21 9 -13.361 3.505 Fused silica 21.5 10 -18.352 0.599 25.5 11
Infinity 0.598 0.8876633 12 -22.089 1.498 Fused silica 1.735372 13
-2.492 5.525 2.742536 14 -25.242 1.750 Fused silica 6.958087 15
-8.752 1.574 7.613493 STO Infinity 1.011 8.782304 17 -26.420 1.750
Fused silica 9.130406 18 -10.453 0.500 9.615398 19 214.479 2.000
Fused silica 10.09149 20 -12.858 0.500 10.245 21 10.710 5.074 Fused
silica 9.775169 22 5.729 3.622 7.468521 23 -6.365 1.499 Fused
silica 7.601525 24 -11.721 0.499 8.660195 25 20.390 2.000 Fused
silica 9.505927 26 -47.176 -15.391 9.654623 27 Infinity 15.391
8.373404 28 Infinity 40.197 9.675574 29 -- 0.000 -- 30 Infinity
6.000 Fused silica 19.30992 31 Infinity 0.000 26.25127 32 -- 0.000
-- 33 Infinity 30.000 17.76485 34 -- 0.000 -- 35 Infinity 6.000
Fused silica 27.58405 36 Infinity 0.000 21.36646 37 -- 0.000 -- 38
Infinity 81.000 15.75755 39 -140.860 4.000 Fused silica 22.67915 40
35.044 1.068 23.39086 41 31.623 9.000 CAF2 24.17115 42 -71.279
1.000 24.64826 43 34.991 8.000 CAF2 24.5185 44 -50.752 1.500
23.4315 45 -37.766 3.000 Fused silica 22.75917 46 331.537 39.138
21.89289 47 11.729 3.402 Fused silica 12.61895 48 9.275 6.254
10.82904 49 -22.713 2.500 Fused silica 10.19172 50 149.521 45.554
10.31249 51 -142.117 5.000 Fused silica 16.06325 52 -25.943 76.816
16.73351 53 -369.224 5.000 CAF2 11.62667 54 -14.234 1.000 11.50051
55 -12.790 2.000 Fused silica 11.04605 56 20.324 1.000 11.08561 57
18.583 5.500 CAF2 11.41199 58 -32.851 38.519 11.39769 59 -- 100.000
5.11369 IMA Infinity 16.29315
[0070] The design spectrum can be limited to 266-365 nm and
reoptimized for a 0.5 mm field size. The tube lens design of FIG.
7B also uses only fused silica and calcium fluoride and has all the
flexibility for reoptimizing presented for the design in FIG.
7A.
[0071] The maximum numerical apertures of the current designs
approaches or exceeds 0.9. The numerical aperture of a design may
be reduced by placing a variable aperture at the aperture stop of
the objective, effectively limiting the illumination and imaging
light angles. It is also possible to control illumination and
imaging angles independently by placing apertures at an external
pupil plane using imaging optics such as the tube lens designs in
FIG. 7A or FIG. 7B. The numerical aperture of the illumination may
be reduced by underfilling the objective aperture with the
illumination light. Such a design enables the full imaging NA to be
used.
[0072] An alternate aspect of the current design is an objective
with increased field size. This aspect of the design is presented
in FIG. 7C. The main difference between the design of FIG. 7C and
that of FIG. 3 is the increase field size from 0.4 mm to 1.0 mm and
an increase in lens diameter from 25 mm to 58 mm. In contrast, this
field diameter is the same as in the design of FIG. 2. Maximum lens
diameter of this design is much smaller than the design of FIG. 2.
The objective of the design of FIG. 7C is corrected over a
bandwidth from 285 to 320 nm, maintains the high 0.90 numerical
aperture, and the worst case polychromatic wavefront error for the
FIG. 7C design is approximately 0.033 waves.
[0073] From FIG. 7C, the catadioptric group 771 includes a Mangin
mirror element 772, which is a reflectively coated lens element,
and a concave spherical reflector 773, which is also a reflectively
coated lens element. Both Mangin mirror element 772 and concave
spherical reflector 773 have central optical apertures where
reflective material is absent. The absence of reflective material,
in the center of the components shown, allows light to pass from
the object or specimen 770 (not shown) through Mangin mirror
element 772, reflect from the second surface of concave spherical
reflector 773 onto the Mangin mirror element 772, and transmit
through concave spherical reflector 773 to form an intermediate
image 790 between concave spherical reflector 773 and field lens
group 774, comprising three field lens elements 783, 784, and 785
in this aspect of the design.
[0074] The focusing lens group 775 employs multiple lens elements,
in this aspect the seven lens elements 776, 779, 780, 781, and 782,
which may all be formed from a single type of material. The
focusing lens group 775 collects light from the field lens group
774, including the intermediate image 790.
[0075] The design presented in FIG. 6 has the advantages and
flexibility described with respect to the design of FIG. 3. The
lens prescription for this design is shown in Table 5.
5TABLE 5 Prescription for lenses for the design of FIG. 7C Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0.000 1
Infinity 43.913 23.946 STO Infinity -43.913 20.000 3 349.851 4.500
Fused silica 23.928 4 -43.383 0.500 23.709 5 30.361 3.650 Fused
silica 21.950 6 16.181 7.177 19.386 7 -17.138 7.305 Fused silica
19.277 8 32.672 0.872 23.722 9 47.511 7.000 Fused silica 23.916 10
-30.308 0.500 25.201 11 37.466 5.500 Fused silica 26.737 12
-147.458 27.319 26.555 13 14.910 4.500 Fused silica 21.011 14
22.738 0.500 19.515 15 20.121 5.000 Fused silica 19.161 16 -127.415
7.984 17.640 17 12.578 2.500 Fused silica 7.187 18 -46.414 0.500
5.333 19 -12.279 3.131 Fused silica 4.668 20 -15.865 2.594 1.955 21
-576.001 2.250 Fused silica 4.516 22 -20.181 0.250 6.277 23 40.385
6.603 Fused silica 60.000 24 29.574 15.917 50.000 25 -777.423
10.056 Fused silica 50.000 26 -202.605 -10.056 MIRROR 50.000 27
-777.423 -15.917 50.000 28 29.574 -6.603 Fused silica 50.000
[0076] Further aspects of the design are presented in FIGS. 8-11,
where FIG. 8 is a design able to perform in the presence of
wavelengths from approximately 311-315 nm, having approximately 26
mm diameter, a field size approximately 0.28 mm, and NA of
approximately 0.90. The lens prescription for this design is shown
in Table 6.
6TABLE 6 Prescription for lenses for the design of FIG. 8 Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0.000 1
Infinity 18.849 8.538 STO Infinity -18.849 7.220 3 6.048 4.786
Fused silica 8.419 4 4.149 1.727 5.777 5 19.860 2.000 Fused silica
5.724 6 -17.207 1.449 5.502 7 -3.955 1.200 Fused silica 5.247 8
-12.991 0.100 5.861 9 10.518 5.617 Fused silica 6.098 10 -15.147
0.100 5.985 11 4.995 2.249 Fused silica 5.701 12 -159.821 0.999
5.037 13 -5.316 4.092 Fused silica 4.659 14 -4.477 0.904 4.116 15
2.448 1.906 Fused silica 2.619 16 4.138 0.248 1.101 17 Infinity
1.501 0.801 18 16.697 2.750 Fused silica 25.240 19 13.901 7.871
22.000 20 -78.318 2.000 Fused silica 22.000 21 -100.000 -2.000
MIRROR 22.000 22 -78.318 -7.871 22.000 23 13.901 -2.750 Fused
silica 22.000 24 16.697 2.750 MIRROR 25.240 25 13.901 7.871 22.000
26 -78.318 2.000 Fused silica 21.000 27 -100.000 0.200 22.000 IMA
Infinity 0.291
[0077] FIG. 9 is an approximately 0.28 mm field design having
approximately 26 mm diameter, a wavelength of between approximately
297 and 313 nm, and NA of approximately 0.90. The lens prescription
for this design is shown in Table 7.
7TABLE 7 Prescription for lenses for the design of FIG. 9 Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0.000 1
Infinity 20.163 8.585 STO Infinity -20.163 7.170 3 -115.896 1.750
Fused silica 8.591 4 -16.723 5.036 8.562 5 -8.430 2.000 Fused
silica 7.122 6 -9.664 0.100 7.349 7 11.608 1.200 Fused silica 7.019
8 4.779 1.598 6.337 9 10.332 1.750 Fused silica 6.622 10 135.162
1.719 6.592 11 -6.281 2.555 Fused silica 6.583 12 -9.052 0.100
7.587 13 5.854 3.250 Fused silica 7.900 14 -17.400 1.125 7.264 15
-7.026 1.499 Fused silica 6.559 16 -8.971 5.055 6.242 17 2.951
1.906 Fused silica 2.442 18 -21.084 0.500 1.255 19 Infinity 1.580
0.314 20 17.135 4.713 Fused silica 26.000 21 12.147 6.064 20.000 22
-164.287 2.500 Fused silica 20.000 23 -100.000 -2.500 MIRROR 20.000
24 -164.287 -6.064 20.000 25 12.147 -4.713 Fused silica 20.000 26
17.135 4.713 MIRROR 26.000 27 12.147 6.064 20.000 28 -164.287 2.500
Fused silica 20.000 29 -100.000 0.200 20.000 IMA Infinity 0.280
[0078] FIG. 10 is an approximately 0.4 mm field design having
approximately 26 mm diameter, a wavelength of between approximately
297 and 313 nm, and NA of approximately 0.90. The lens prescription
for this design is shown in Table 8.
8TABLE 8 Prescription for lenses for the design of FIG. 10 Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0.000 1.000
Infinity 17.977 8.974 STO Infinity -17.977 7.171 3.000 -73.415
1.750 Fused silica 8.988 4.000 -16.484 3.889 8.954 5.000 -7.914
3.077 Fused silica 7.822 6.000 -8.792 0.103 8.317 7.000 10.984
1.200 Fused silica 7.777 8.000 4.966 1.460 6.942 9.000 9.494 1.500
Fused silica 7.137 10.000 23.256 2.020 7.037 11.000 -6.669 1.871
Fused silica 7.044 12.000 -10.034 0.100 7.866 13.000 6.034 2.500
Fused silica 8.344 14.000 66.970 0.100 7.904 15.000 12.304 1.750
Fused silica 7.531 16.000 -60.162 1.300 6.846 17.000 -6.852 1.499
Fused silica 6.139 18.000 -8.993 4.511 5.804 19.000 3.141 1.750
Fused silica 2.466 20.000 -15.561 0.499 1.420 21.000 Infinity 1.841
0.794 22.000 17.138 4.708 Fused silica 26.000 23.000 12.005 6.070
20.000 24.000 -177.009 2.500 Fused silica 20.000 25.000 -100.000
-2.500 MIRROR 20.000 26.000 -177.009 -6.070 20.000 27.000 12.005
-4.708 Fused silica 20.000 28.000 17.138 4.708 MIRROR 26.000 29.000
12.005 6.070 20.000 30.000 -177.009 2.500 Fused silica 20.000
31.000 -100.000 0.200 20.000 IMA Infinity 0.401
[0079] FIG. 11 illustrates a broad band design having approximately
26 mm diameter, a wavelength of between approximately 266 and 313
nm, field size of approximately 0.28 mm, and NA of approximately
0.90. The lens prescription for this design is shown in Table
9.
9TABLE 9 Prescription for lenses for the design of FIG. 11 Surf
Radius Thickness Glass Diameter OBJ Infinity Infinity 0.000 1.000
Infinity 19.109 8.783 STO Infinity -19.109 7.500 3.000 59.725 1.500
F_SILICA 8.772 4.000 -337.579 1.500 8.650 5.000 -9.464 1.500
F_SILICA 8.574 6.000 -9.415 4.925 8.900 7.000 8.637 1.200 F_SILICA
7.651 8.000 4.897 2.128 6.903 9.000 214.349 1.750 F_SILICA 7.117
10.000 -12.598 1.147 7.334 11.000 -7.560 1.000 F_SILICA 7.320
12.000 -772.023 0.100 7.974 13.000 9.411 2.000 F_SILICA 8.548
14.000 -56.012 0.099 8.529 15.000 7.107 2.750 F_SILICA 8.352 16.000
-22.495 1.159 7.805 17.000 -7.960 1.499 F_SILICA 7.103 18.000
-10.073 5.482 6.716 19.000 3.034 1.748 F_SILICA 2.380 20.000
-20.121 0.245 1.276 21.000 Infinity 1.041 0.955 22.000 16.855 4.806
F_SILICA 26.000 23.000 11.392 6.422 20.000 24.000 -133.502 2.000
F_SILICA 20.000 25.000 -100.000 -2.000 MIRROR 20.000 26.000
-133.502 -6.422 20.000 27.000 11.392 -4.806 F_SILICA 20.000 28.000
16.855 4.806 MIRROR 26.000 29.000 11.392 6.422 20.000 30.000
-133.502 2.000 F_SILICA 20.000 31.000 -100.000 0.200 20.000 IMA
Infinity 0.283
[0080] The current invention is capable of similar or better
performance over previously known catadioptric objectives with
smaller maximum lens diameters. The lens having the largest
diameter in these designs is typically the highly curved Mangin
mirror element, the second optical element from the object or
specimen.
[0081] Designs similar to the objective shown in FIG. 2 have 0.9
NA, large corrected bandwidths using large optical elements, and
relatively tight manufacturing tolerances. The current designs
presented above display large corrected bandwidths but use
relatively small optical elements, and the designs have loose
manufacturing tolerances. FIG. 12 is a graph contrasting previous
systems against the current design in terms of relative bandwidth
and maximum lens diameter. Relative bandwidth is defined as the
bandwidth of the objective divided by the center wavelength.
Previous systems are well corrected for relative bandwidths of at
least 0.5 using lenses with maximum diameters greater than 100 mm.
Current objective designs as presented herein use a single glass
material and are self corrected up to approximately 0.16 using
lenses with maximum diameters from around 20 mm up to 100 mm.
Further correction of these objectives over relative bandwidths up
to 0.5 are possible using tube lenses to correct residual chromatic
aberrations as in the designs of FIGS. 7A and 7B. Similar
correction is also possible for the objective alone by restricting
NA or field size requirements.
[0082] FIG. 13 is a graph contrasting previous designs and the
present design in terms of field size and maximum lens diameter.
Previous systems tend to be well corrected for field sizes of 1 mm
using lenses with maximum diameters greater than 100 mm. Current
objectives using the designs presented herein are corrected for
field sizes of 0.4 mm using lenses with maximum diameters from
around 25 mm, and 1.0 mm field sizes using lens diameters of 58 mm.
From this and the graph of FIG. 12, the ratio between field size
and diameter of the largest element (including the Mangin mirror
arrangement, field lens(es), and focusing lens(es), is generally
less than 100 to 1, and may be less than 60 to 1. For example, the
58 mm lens diameter versus the 1.0 mm field size produces a ratio
of 58 to 1. Larger field sizes are also possible with increasing
lens diameter. Further correction of these objectives over larger
field sizes are possible using tube lenses to correct residual
chromatic aberrations as in the designs of FIGS. 7A and 7B. Similar
correction is also possible for the objective alone by restricting
NA or bandwidth requirements.
[0083] The present design may be employed in various environments,
including but not limited to lithography, microscopy, biological
inspection, medical research, and the like.
[0084] The design presented herein and the specific aspects
illustrated are meant not to be limiting, but may include alternate
components while still incorporating the teachings and benefits of
the invention, namely the small design having a high NA able to be
employed in various wavelengths using different illumination modes.
While the invention has thus been described in connection with
specific embodiments thereof, it will be understood that the
invention is capable of further modifications. This application is
intended to cover any variations, uses or adaptations of the
invention following, in general, the principles of the invention,
and including such departures from the present disclosure as come
within known and customary practice within the art to which the
invention pertains.
* * * * *